The Enterprise is on a peaceful mission at Starbase 12 when a bizarre cosmic phenomenon causes a Klingon ship to suddenly vanish - with Spock aboard for the ride. Spock's last message from the Klingon ship is cryptic and frightening. The Klingons are traveling into the past, searching for the one man who holds the key to the future. If they can kill that man, the course of history will be changed - and the Federation will be destroyed!
The crew of the Nautilus, a battered Starship of mysterious origin, is beamed aboard the Enterprise. The group claims they are freedom fighters from the future working to save the Federation from the Consilium - a group of corrupt power-seekers. But when the Nautilus crew members suddenly seize control of the Enterprise, and a Starship from the future arrives to arrest the renegades, Kirk must separate his true allies from those who wish to destroy the Federation.
Unwilling to embrace any form of technology, the Midgwins have exhausted their world's natural resources, and stand on the brink of global famine. When Captain Kirk and the Enterprise arrive to aid the Midgwins, they find themselves caught up in that race's struggle for survival ... a struggle whose climactic battle pits them against a creature of darkness and shadow.